The present invention relates to a system and method for treating sleep apnea, and more particularly to a system and method for treatment of obstructive sleep apnea using implantable microstimulators.
Sleep apnea is the inability to breath while sleeping. The most common cause is a mechanical obstruction of the airway. This can arise from a variety of causes acting at a variety of sites, as described in detail by the report of Dr. Frances J. R. Richmond (attached hereto as Appendix A, which Appendix A is incorporated herein by reference). At each of these sites, various muscles are available whose mechanical action can be used to open the airway. Thus, in principle, it should be possible to stimulate these muscles electrically at appropriate times during sleep to produce contractions that maintain patency or open an obstructed airway. It may also be possible to produce muscle contractions via reflexes. Sensory axons that are present in the muscle nerves or in separate nerves supplying skin or mucous membranes may have connections in the spinal cord or brain stem to the motor neurons that control these muscles. Electrical stimulation of these sensory axons would then result in the desired muscle contractions.
Unfortunately, the muscles that control the airway and the nerves that supply them are, for the most part, located deep in the neck and oropharynx, adjacent to many vital and delicate structures. The present invention describes an approach in which very small electronic devices can be implanted with minimal surgical intervention in order to control these muscles to prevent or interrupt sleep apnea without disturbing the sleeping patient.
Obstructive sleep apnea (OSA) is characterized by frequent periods of airway occlusion during sleep, with concomitant obstruction of inspiratory airflow, drop in blood oxygen and interruption of sleep when the patient awakes to use voluntary muscle contraction to open the airway and take a few deep breaths. The mechanical locations and structural causes of obstruction are multiple. The most frequent mechanisms include settling of the tongue, uvula, soft palate or other tissues against the airway during the negative pressure associated with inspiration. This may be related to adipose tissue accumulation, lack of muscle tone or inadequate central respiratory drive to the tongue and/or other accessory respiratory muscles around the oropharyngeal airway.
Current treatments for OSA include behavioral control of sleep posture (e.g. sewing a tennis ball in the back of a pajama shirt), positive airway pressure applied via a face mask and ventilating pump, and surgical reduction of the soft tissues in the airway. Disadvantageously, patient compliance with noninvasive methods is low. Clinical success rates with surgery are mixed because the exact mechanical problem is often unclear, the extent of the surgery is limited for practical reasons, and the soft tissue may regrow.
Recently, a fully implanted system using functional electrical stimulation has been developed in a collaboration between Johns Hoplins University and Medtronic Corp. It includes an implanted pressure sensor, electronic controller and a nerve cuff electrode on the genioglossal nerve, which is used to stimulate the motor axons that control tongue protrusion. The onset of each inspiratory phase is detected by sensing the negative pressure wave under the manubrium of the sternum, which triggers a preset train of stimuli that causes the tongue muscles to contract, lifting it away from the posterior oropharynx to prevent occlusion from occurring. In a small clinical trial, this system had some success, but it is highly invasive and prone to complications from the surgical placement of the sensor and nerve cuff. Furthermore, the algorithm for detecting the onset of inspiration is complex and prone to error as a result of mechanical artefacts from cardiac pulsation and postural movements.
Electrical stimulation of muscles with intact motor supply, such as in OSA patients, typically involves activation of the large diameter axons of the motor neurons that, in turn, activate the muscle fibers to contract. Electrical stimulation may be achieved by several routes. Transcutaneous electrical and magnetic stimulation are noninvasive but relatively nonselective, usually producing substantial activation of cutaneous sensory nerves that results in intrusive and often disagreeable sensations. Nerve trunk stimulation via surgically implanted nerve cuff or epineural electrodes stimulates both the motor and sensory axons that run in those nerves. The largest diameter fibers of both types tend to be excited first and there is little conscious sensation associated with the large diameter proprioceptive fibers in muscle, as opposed to cutaneous, nerves. However, it is thus difficult to achieve reliable stimulation of only the largest fibers because the recruitment curves are relatively steep. Muscle nerves do contain smaller diameter sensory fibers that signal pain, pressure and other undesirable sensations. Furthermore, many muscle nerves supply axons to multiple muscles or compartments of muscles that may have different and perhaps undesirable actions. For example, the more proximal and surgically accessible portion of the genioglossal nerve supplies portions of the tongue muscles that actually retract rather than protrude the tongue. Intramuscular wires tend to recruit motor and proprioceptive axons in only the compartment of muscle in which they are located, but they are difficult to implant and maintain without migration or breakage as a result of the constant motion of the muscle and the traction of attached leads. It is thus seen that there is a need for a better vehicle for treating obstructive sleep apnea than has heretofore been available.
Respiration can be viewed as an oscillator whose frequency is determined by several different neural and mechanical control loops involving metabolic rate, neural sensing of oxygen and carbon dioxide levels in the blood stream, respiratory muscle activity, airway impedance and tidal volume. Individuals with normal airways tend to have fairly regular respiratory rhythms punctuated by occasional irregularities such as deep sighs that help to clear poorly ventilated portions of the lung and keep alveoli inflated. Conscious and semiconscious individuals can be ventilated artificially by a respirator that applies positive pressure to force air into the lungs at a regular interval, but it is often difficult to match the ventilation so applied to the perceived need for air by the patient and his/her nervous system, which may result in the patient "fighting" the ventilator. Nevertheless, biological oscillators are easily entrained to periodic external events as long as the frequency of those events is sufficiently close to the natural period of the biological oscillator. This is a general property of systems of loosely coupled oscillators, as originally described mathematically by Arthur T. Winfree (The Geometry of Biological Time, Springer Study Edition, 1991) and may also reflect plastic properties of the nervous system that underlie learning and adaptive control for many sensorimotor behaviors.